Caterpillar Brake Systems Troubleshooting PDF

GLOBAL DEALER LEARNING Brake Systems Troubleshooting Air Brake Systems MODULE 2 - TEXT REFERENCE SERVxxxx © 2024 Caterpillar. All Rights Reserved. CAT, CATERPILLAR, LET’S DO THE WORK, their respective logos, “Caterpillar Corporate Yellow”, the “Power Edge” and Cat “Modern Hex” trade dress as we...

GLOBAL DEALER LEARNING Brake Systems Troubleshooting Air Brake Systems MODULE 2 - TEXT REFERENCE SERVxxxx © 2024 Caterpillar. All Rights Reserved. CAT, CATERPILLAR, LET’S DO THE WORK, their respective logos, “Caterpillar Corporate Yellow”, the “Power Edge” and Cat “Modern Hex” trade dress as well as corporate and product identity used herein, are trademarks of Caterpillar and may not be used without permission. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -2- Module 2 - Air Brake Systems TABLE OF CONTENTS SAFETY BRIEFING 4 PURPOSE 5 REASON 6 ASSESSMENT CRITERIA 7 LEARNING OUTCOMES 8 LEARNING OUTCOMES (CONT.) 9 AIR BRAKE SYSTEMS TROUBLESHOOTING INTRODUCTION 10 AIR BRAKE SYSTEMS OVERVIEW 11 SERVICE BRAKE CIRCUIT (MG) 12 PARKING BRAKE CIRCUIT (MG) 13 SERVICE BRAKE CIRCUIT (WTS) 14 PARKING BRAKE CIRCUIT (WTS) 16 SERVICE BRAKE CIRCUIT (AT) 17 PARKING BRAKE CIRCUIT (AT) 19 SERVICE BRAKE CIRCUIT (MWL) 20 PARKING BRAKE CIRCUIT (MWL) 22 SERVICE BRAKE AIR CIRCUIT (OHT) 23 SERVICE BRAKE HYDRAULIC CIRCUIT (OHT) 24 PARKING BRAKE CIRCUIT (OHT) 25 SYSTEM TESTING AND ADJUSTING OVERVIEW 27 OPERATIONAL CHECKS: BRAKE HOLDING ABILITY TEST 28 OPERATIONAL CHECKS: LEAK CHECK 30 OPERATIONAL CHECKS: AIR PRESSURE GAUGE TEST 32 OPERATIONAL CHECKS: AIR COMPRESSOR GOVERNOR TEST 33 OPERATIONAL CHECKS: AIR COMPRESSOR GOVERNOR ADJUSTMENT 34 OPERATIONAL CHECKS: LOW BRAKE AIR PRESSURE SWITCH TEST 35 OPERATIONAL CHECKS: SYSTEM PROTECTION VALVE TEST (AT) 36 OPERATIONAL CHECKS: PARKING BRAKE AIR PRESSURE TEST 37 OPERATIONAL CHECKS: SERVICE BRAKE AIR PRESSURE AND OIL PRESSURE TEST 38 OPERATIONAL CHECKS: HYDRAULIC CYLINDER STROKE TEST 39 OPERATIONAL CHECKS: SLACK ADJUSTER TEST 40 Caterpillar: Confidential Yellow SERVxxxx - 12/24 -3- Module 2 - Air Brake Systems OPERATIONAL CHECKS: PARKING BRAKE OIL PRESSURE TEST 41 SYSTEM TROUBLESHOOTING OVERVIEW 42 CATERPILLAR 7-STEP DIAGNOSTIC PROCESS 43 SYSTEM TROUBLESHOOTING: INITIAL INSPECTION 45 SYMPTOM TROUBLESHOOTING 46 SYSTEM TROUBLESHOOTING: COMPRESSOR 47 SYSTEM TROUBLESHOOTING: GOVERNOR 48 SYSTEM TROUBLESHOOTING: AIR DRYER 49 SYSTEM TROUBLESHOOTING: AIR RESERVOIR 50 SYSTEM TROUBLESHOOTING: SERVICE BRAKE CONTROL VALVE 51 SYSTEM TROUBLESHOOTING: SERVICE BRAKES 52 SYSTEM TROUBLESHOOTING: BRAKE CYLINDER 53 SYSTEM TROUBLESHOOTING: SLACK ADJUSTER 54 SYSTEM TROUBLESHOOTING: PARKING BRAKE CONTROL VALVE 55 SYSTEM TROUBLESHOOTING: PARKING BRAKES 56 PURPOSE REVIEW 57 REASON REVIEW 58 ASSESSMENT CRITERIA REVIEW 59 LEARNING OUTCOMES REVIEW 60 LEARNING OUTCOMES REVIEW (CONT.) 61 MODULE CONCLUSION 62 Caterpillar: Confidential Yellow SERVxxxx - 12/24 -4- Module 2 - Air Brake Systems SAFETY BRIEFING Emergency Phone Numbers First Aid Responders Location of Exits Location of Fire Extinguisher Room Alerts or Hazards Designated Location for Evacuation Storm Shelter Hazardous Material Safely Home. Everyone. Every Day.™ 4 SAFETY BRIEFING Before beginning this module, the following topics will be reviewed: Emergency Phone Numbers First Aid Responders Location of Exits Location of Fire Extinguisher Room Alerts or Hazards Designated Location for Evacuation Storm Shelter Hazardous Material Safely Home. Everyone. Every Day.™ NOTE: Any system or component specifications stated in this training module are used only for training purposes. Components and specifications may change as the product evolves. Always refer to Caterpillar Service information to obtain the most current specifications when servicing Cat products. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -5- Module 2 - Air Brake Systems PURPOSE This module covers air brake system components, operation, testing, and troubleshooting procedures for machines equipped with air brake systems. 5 PURPOSE Explain the purpose of this This module covers air brake system components, operation, testing, and module. troubleshooting procedures for machines equipped with air brake systems. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -6- Module 2 - Air Brake Systems REASON Successful completion validates the knowledge, skills, and behaviors that are required to properly diagnose and repair air brake systems used in Cat® machines. 6 REASON Explain the reason for this Successful completion validates the knowledge, skills, and behaviors that are module. required to properly diagnose and repair air brake systems used in Cat machines. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -7- Module 2 - Air Brake Systems ASSESSMENT CRITERIA Knowledge Assessment Closed book Minimum passing score of 80% Skills Assessment Competent/Not Yet Competent or Instructor Observation Test procedures are performed correctly 7 ASSESSMENT CRITERIA Explain the assessment criteria After the completion of this module, the participant must complete a knowledge for this module. assessment with a minimum passing score of 80%. The instructor will determine whether the participant is competent or not yet competent to perform the air brake tests. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -8- Module 2 - Air Brake Systems LEARNING OUTCOMES After completing this Air Brake Systems module of the Brake Systems Troubleshooting course you should be able to: Identify air brake systems components. Explain air brake systems operation. Describe the different applications of air brake systems. Describe air brake systems testing and adjusting procedures. Describe air brake systems troubleshooting procedures. Perform air brake systems testing and adjusting procedures. 8 LEARNING OUTCOMES Explain the learning outcomes of After the completion of this module, the participant will be able to: this module. Identify air brake systems components. Explain air brake systems operation. Describe the different applications of air brake systems. Describe air brake systems testing and adjusting procedures. Describe air brake systems troubleshooting procedures. Perform air brake systems testing and adjusting procedures. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -9- Module 2 - Air Brake Systems LEARNING OUTCOMES (CONT.) After completing this Air Brake Systems module of the Brake Systems Troubleshooting course you should be able to: Perform air brake systems troubleshooting procedures. 9 LEARNING OUTCOMES (CONT.) Explain the learning outcomes of After the completion of this module, the participant will be able to: this module. Perform air brake systems troubleshooting procedures. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -10- Module 2 - Air Brake Systems AIR BRAKE SYSTEMS TROUBLESHOOTING INTRO 10 AIR BRAKE SYSTEMS TROUBLESHOOTING INTRODUCTION Introduction to air brake systems This module is an introduction to air brake systems troubleshooting. troubleshooting. With knowledge of the correct operation and testing procedures, the technician should be able to take the next step of identifying a problem when the air brake system is not operating correctly. Additional theory (e.g., advanced systems operation) will be presented where necessary to ensure a complete understanding of the air brake systems. NOTE: The Caterpillar 7-Step Diagnostic Process was introduced in the basic service curriculum and will be referred to throughout the advanced courses to promote an organized method of problem solving. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -11- Module 2 - Air Brake Systems AIR BRAKE SYSTEMS HEADER OVERVIEW All Air Brake System Air Over Hydraulic Brake System 11 AIR BRAKE SYSTEMS OVERVIEW Give an overview of air brake This section covers the system operation of various air brake systems that are systems. currently used or have been used on Cat machines. Air brake systems discussed include: All Air Brake System Air brake system that uses a type of air brake actuator that engages or releases the brakes using only air pressure. Air Over Hydraulic Brake System Air brake system that uses a type of hydraulic brake actuator to engage the brakes. These systems are actuated by air pressure but engaged by hydraulic oil pressure. After completing this section, you should be able to: Identify air brake system components. Explain air brake system operation. Describe the different applications of air brake systems. NOTE: For detailed information on the air charging system, refer to SERV2264 Module 3: Air Brake Systems Introduction. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -12- Module 2 - Air Brake Systems SERVICE BRAKE CIRCUIT (MG) 3 4 2 2 1 4 3 1 4 4 Pneumatic Air Pressure High Oil Pressure (Brake Application) Low Oil Pressure (Return to Tank) 12 1. Air reservoir 3. Brake pedal 2. Service brake control valve 4. Brake actuators SERVICE BRAKE CIRCUIT (MG) Identify the service brake circuit The air brake system on a motor grader (MG) uses air pressure to actuate and on a Motor Grader (MG). engage the service brakes and the parking brakes. In this example, the brakes are activated by independent circuits (right side and left side). Air pressure compresses the multiple disc brakes to slow or stop the machine. In the service brake circuit, air flows from the air reservoir (1) to the service brake control valve (2). When the brake pedal (3) is depressed, the brake control valve directs air to the brake actuators (4). The air pressure in the brake actuators engages the service brakes. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -13- Module 2 - Air Brake Systems PARKING BRAKE CIRCUIT (MG) 3 1 5 2 1 4 4 Pneumatic Air Pressure High Oil Pressure (Brake Application) Low Oil Pressure (Return to Tank) 13 1. Parking brake control valve 4. Brake actuator 2. Service brake control valve 5. Air reservoir 3. Transmission control lever PARKING BRAKE CIRCUIT (MG) Identify the parking brake circuit In this MG air brake system example, the parking brake is a multiple disc assembly on a Motor Grader (MG). mounted to the transmission and when engaged keeps the transmission output shaft from turning. In the parking brake circuit, air flows from the air reservoir to the parking brake control valve (1), via the service brake control valve (2). When the transmission control lever (3) in the cab is placed in the parking brake ENGAGED position, the parking brake control valve blocks air from flowing to the brake actuator (4). The spring force in the brake actuator engages the parking brakes. NOTE: Callouts not mentioned in text explanations for this illustration, and those of subsequent illustrations, are shown for reference purposes only. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -14- Module 2 - Air Brake Systems SERVICE BRAKE CIRCUIT (WTS) 15 2 1 10 5 13 12 11 14 16 16 3 7 9 4 8 6 2 1 14 1. Tractor brake actuators 9. Tractor double check valve 2. Scraper brake actuators 10. Scraper double check valve 3. Dual brake chamber 11. Parking brake piston 4. Tractor air reservoirs 12. Service brake diaphragm 5. Service brake control valve 13. Pressure protection valve 6. Tractor relay valve 14. Quick release and holding valves 7. Scraper air reservoir 15. Service brake chamber 8. Scraper relay valve 16. Parking brake solenoid valves SERVICE BRAKE CIRCUIT (WTS) Identify the service brake The air brake system on a wheel-tractor scraper (WTS) uses air pressure to circuit on a Wheel-Tractor actuate and engage the service brakes and the parking brakes. Scraper (WTS). The tractor brake actuators (1) and the scraper brake actuators (2) are activated by independent circuits. A dual brake chamber (3) is used to engage the service and parking brakes. The upper chamber actuates the service brakes and the lower chamber actuates the parking brakes. In the service brake circuit, air flows from the tractor air reservoirs (4) to the service brake control valve (5) and to the tractor relay valve (6). Air also flows from the scraper air reservoir (7) to the scraper relay valve (8). When the brake pedal is depressed, the brake control valve directs pilot air pressure to the relay valves. The relay valves then direct air pressure from the air reservoirs to the brake actuators. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -15- Module 2 - Air Brake Systems Air also flows from the brake control valve to the tractor double check valve (9) and from the scraper relay valve to the scraper double check valve (10). The double check valves prevent the parking brake piston (11) and the service brake diaphragm (12) in the brake actuator from being activated at the same time. The service brakes will release the parking brakes if the parking brakes are engaged when the service brake pedal is depressed. This is necessary to ensure that the parking brake and service brake are not applied at the same time, which could damage the brake components. NOTE: The WTS air system is equipped with a pressure protection valve (13). The pressure protection valve controls air flow to the accessory air circuits. Air will not flow to the accessory circuits until there is sufficient air pressure to operate the brake circuits. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -16- Module 2 - Air Brake Systems PARKING BRAKE CIRCUIT (WTS) 11 3 7 10 4 8 9 5 2 2 6 1 4 1 3 16 1. Air reservoirs 7. Service brake control valve 2. Parking brake solenoid valves 8. Pressure protection valve 3. Brake actuators 9. Parking brake piston 4. Double check valves 10. Service brake diaphragm 5. Quick release and holding valves 11. Service brake chamber 6. Parking brake chamber PARKING BRAKE CIRCUIT (WTS) Identify the parking brake In the WTS parking brake circuit, air flows from the air reservoirs (1) to the parking circuit on a Wheel-Tractor brake solenoid valves (2). When the parking brake knob in the cab is pulled out, the Scraper (WTS). ECM DE-ENERGIZES the solenoid valves, which block air from flowing to the brake actuators (3). The spring force in the brake actuators engages the parking brakes. When the parking brake knob in the cab is pushed in, the ECM ENERGIZES the solenoid valves, which open and allow air to flow from the air reservoirs through the double check valves (4) and the quick release and holding valves (5) to the parking brake chamber (6) in the brake actuators. The air fills the parking brake chamber and compresses the parking brake springs, which retracts the actuator rods and releases the parking brakes. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -17- Module 2 - Air Brake Systems SERVICE BRAKE CIRCUIT (AT) 3 6 7 13 5 12 5 14 8 10 11 1 15 9 2 3 D400D Articulated Truck 4 17 1. Hydraulic brake actuator 9. Shuttle valve 2. Front brakes 10. Quick release valve 3. Dual brake chambers 11. Shuttle valve 4. Air compressor 12. Parking brake control valve 5. Three air reservoirs 13. Parking brake air reservoir 6. Air dryer 14. Quick release and holding valves 7. System protection valve 15. Relay valve 8. Service brake control valve SERVICE BRAKE CIRCUIT (AT) Identify the service brake circuit In many articulated trucks (AT), the front service brakes are actuated by an air over on an Articulated Truck (AT). hydraulic brake system and the rear service brakes and parking brakes are actuated and engaged by air pressure. An air over hydraulic brake actuator (1) uses air pressure to move a hydraulic piston and oil pressure is used to engage the front brakes (2). A dual brake chamber (3) is used to engage the rear service brakes and parking brakes. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -18- Module 2 - Air Brake Systems The air compressor (4) supplies oil to the three air reservoirs (5), via the air dryer (6) and system protection valve (7). Air flows from the three air reservoirs to the service brake control valve (8). When the brake pedal is depressed, the brake control valve directs air pressure to the shuttle valve (9), which shifts and allows air to flow to the hydraulic brake actuator to engage the front brakes using oil pressure. The service brake control valve also directs air pressure to the quick release valve (10), which then directs the air pressure to the rear brake actuators. Air also flows from the brake control valve to the shuttle valve (11), which prevents the parking brake and the service brake from being activated at the same time. The service brakes will release the parking brakes if the parking brakes are engaged when the service brake pedal is depressed. This is necessary to ensure that the parking brake and service brake are not applied at the same time, which could damage the brake components. NOTE: The AT air system is equipped with a system protection valve, which separates the air supply circuits. The separate supply for each circuit protects the circuits if one circuit develops a leak. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -19- Module 2 - Air Brake Systems PARKING BRAKE CIRCUIT (AT) 9 10 1 8 3 2 6 11 8 7 13 5 12 14 3 D400D Articulated Truck 4 19 1. Parking brake air reservoir 8. Three air reservoirs 2. Parking brake control valve 9. Air dryer 3. Dual brake chambers 10. System protection valve 4. Air compressor 11. Service brake control valve 5. Relay valve 12. Shuttle valves 6. Quick release and holding valves 13. Hydraulic brake actuator 7. Quick release valve 14. Front brakes PARKING BRAKE CIRCUIT (AT) Identify the parking brake In an AT parking brake circuit, air flows from the parking brake air reservoir (1) to circuit on an Articulated the parking brake control valve (2). When the parking brake lever in the cab is Truck (AT). activated the parking brake control valve blocks air from flowing to the brake actuators. The spring force in the brake actuators engages the parking brakes. When the parking brake lever in the cab is deactivated, the parking brake control valve directs pilot air pressure through the shuttle valve (4) to the relay valve (5). The relay valve then directs air flow from the air reservoir to the quick release and holding valves (6) and to the parking brake chamber (3) in the brake actuators. The air fills the parking brake chambers and compresses the parking brake springs, which retracts the actuator rods and releases the parking brakes. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -20- Module 2 - Air Brake Systems SERVICE BRAKE CIRCUIT (MWL) 8 9 2 3 5 7 1 4 7 6 (Rear) (Front) 966F Series Medium Wheel Loader 20 1. Hydraulic brake actuators 6. Quick release valve 2. Air reservoir 7. Front and rear service brakes 3. Service brake control valves 8. Parking brake actuator 4. Brake pedals 9. Parking brake control valve 5. Double check valve SERVICE BRAKE CIRCUIT (MWL) Identify the service brake In many medium wheel loaders (MWL), the front and rear service brakes are circuit on a Medium Wheel actuated by an air over hydraulic brake system and the parking brake is actuated Loader (MWL). and engaged by air pressure. Air over hydraulic brake actuators (1) use air pressure to move a hydraulic piston and oil pressure is used to engage the multiple disc service brakes. Air flows from the air reservoir (2) to the service brake control valves (3). When the left and right brake pedals (4) are depressed, the brake control valves direct air pressure to the double check valve (5), which prevents air from entering the opposite brake control valve. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -21- Module 2 - Air Brake Systems The air then flows to the quick release valve (6), which directs the air pressure to the hydraulic brake actuators to engage the front and rear service brakes (7) using oil pressure. The quick release valve rapidly vents the air from the brake actuators when the wheel brakes are RELEASED. NOTE: In this MWL example, both the left and right brake pedals are used to engage the service brakes, however, the left brake pedal is also used to neutralize the transmission. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -22- Module 2 - Air Brake Systems PARKING BRAKE CIRCUIT (MWL) 3 2 1 8 5 7 9 4 7 6 (Rear) (Front) 966F Series Medium Wheel Loader 22 1. Air reservoir 6. Quick release valve 2. Parking brake control valve 7. Front and rear service brakes 3. Parking brake actuator 8. Service brake control valves 4. Brake pedals 9. Hydraulic brake actuators 5. Double check valve PARKING BRAKE CIRCUIT (MWL) Identify the parking brake In the MWL parking brake circuit, air flows from the air reservoir (1) to the parking circuit on a Medium Wheel brake control valve (2). When the parking brake knob in the cab is pulled out, the Loader (MWL). parking brake control valve blocks air from flowing to the brake actuator (3). The spring force in the brake actuator engages the parking brake. The parking brake is a shoe type brake assembly mounted to the loader frame and when engaged, keeps the transmission output shaft from turning. When the parking brake lever in the cab is pushed in, the parking brake control valve directs air pressure to the parking brake actuator. The air fills the parking brake chamber and compresses the parking brake springs, which retracts the actuator rod and releases the parking brake. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -23- Module 2 - Air Brake Systems SERVICE BRAKE AIR CIRCUIT (OHT) 4 2 5 3 6 1 23 1. Hydraulic brake actuators 4. Relay valve 2. Air reservoir 5. Double check valve 3. Service brake control valve 6. Retarder valve SERVICE BRAKE AIR CIRCUIT (OHT) Identify the service brake air In off-highway trucks (OHT), the service brakes and parking brakes are actuated by circuit on an Off-Highway Truck an air over hydraulic brake system. The multiple disc brakes in off-highway trucks (OHT). function as service and parking brakes. In the service brake circuit, air over hydraulic brake actuators (1) use air pressure to move a hydraulic piston and oil pressure is used to engage the service brakes. Air flows from the air reservoir (2) to the service brake control valve (3) and to the relay valve (4). When the brake pedal is depressed, the brake control valve directs air pressure to the double check valve (5), which prevents air from entering the retarder valve (6). The brake control valve directs pilot air pressure to the relay valve. The relay valve then directs air pressure from the air reservoir to the top of the brake actuators. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -24- Module 2 - Air Brake Systems SERVICE BRAKE HYDRAULIC CIRCUIT (OHT) 1 4 4 1 2 2 3 3 Pneumatic Air Pressure High Oil Pressure (Brake Application) Low Oil Pressure (Return to Tank) 24 1. Brake cylinders 3. Service brakes 2. Slack adjuster 4. Makeup oil tank SERVICE BRAKE HYDRAULIC CIRCUIT (OHT) Identify the service brake With the service brake pedal depressed, air pressure is present at the top of the hydraulic circuit on an brake cylinder (1). The air pressure moves a piston down, which compresses the Off-Highway Truck (OHT). oil in the bottom of the cylinder. The brake cylinder supplies compressed oil to a slack adjuster (2), which directs the oil to the service brakes (3). Slack adjusters compensate for brake disc wear by allowing a small volume of oil to flow through the slack adjuster and remain between the slack adjuster and the brake piston under low pressure. The slack adjusters maintain a slight pressure on the brake piston at all times. The makeup oil tank (4) supplies oil for the brake cylinders. Oil from the brake cooling circuit provides a continuous supply of oil to the makeup oil tank. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -25- Module 2 - Air Brake Systems PARKING BRAKE CIRCUIT (OHT) 5 6 7 1 3 2 4 25 1. Parking brake release valve 5. Parking brake switch 2. Parking brake 6. Parking brake release pump 3. Parking brake air reservoir 7. Secondary brake lever 4. Secondary brake control valve PARKING BRAKE CIRCUIT (OHT) Identify the parking brake circuit In the OHT parking brake circuit, the parking brake release valve (1) uses air on an Off-Highway Truck (OHT). pressure to move a hydraulic valve spool, which directs oil pressure to release the parking brakes (2). Air flows from the parking brake air reservoir (3) to the secondary brake control valve (4) and parking brake switch (5). Supply oil flows from the parking brake release pump (6) to the parking brake release valve. When the parking brake switch in the cab is moved to the ENGAGED position, air flow is blocked from flowing to the parking brake release valve. The parking brake release valve blocks oil flow to the parking brakes and spring force engages the parking brakes. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -26- Module 2 - Air Brake Systems When the parking brake switch in the cab is moved to the RELEASED position, air flow is directed to the parking brake release valve. The parking brake release valve then directs oil flow to the parking brake and the parking brakes are released. NOTE: Normally, air flows through the secondary brake valve and parking brake valve to the parking brake release valve. When the secondary brake lever (7) is activated, the secondary brake valve blocks air flow to the parking brake release valve, allowing spring force to ENGAGE the parking brakes. The secondary brake valve can be used to modulate parking brake engagement by metering the amount of air flow to the parking brake release valve. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -27- Module 2 - Air Brake Systems SYSTEM TESTING AND ADJUSTING OVERVIEW 27 SYSTEM TESTING AND ADJUSTING OVERVIEW Give an overview of brake system Brake system testing may need to be performed to troubleshoot an air brake testing and adjusting. system problem or to optimize the performance of a machine’s braking system, such as during a Technical Analysis 2 (TA2) inspection. A TA2 inspection may be performed annually or when necessary to restore equipment back to the factory performance specifications. This section identifies and describes the common air brake system operational and pressure tests that are routinely performed to troubleshoot a problem or during a TA2 inspection. After completing this section, you should be able to: Identify air brake system pressure test locations Describe air brake system test procedures Describe air brake system adjusting procedures NOTE: TA2 inspection forms can be located in the Service Information System (SIS) or at https://dealer.cat.com. Refer to the appropriate Testing and Adjusting manual for all of the brake system testing procedures specific to the machine being serviced. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -28- Module 2 - Air Brake Systems BRAKE HOLDING ABILITY TEST Service Brake Parking Brake 28 OPERATIONAL CHECKS: BRAKE HOLDING ABILITY TEST Explain the general procedure The brake holding tests will determine if the service brakes and parking brake are for performing the service brake operating within specification. and parking brake holding ability tests. If the machine moved during either brake test, further inspection and testing may be required. Service Brake Holding Ability Test The general procedure for the service brake holding ability test involves: 1. Start the engine, allow air pressure to reach the normal operating range, raise the implement slightly, and release the parking brake. 2. Put the transmission control in the gear specified by the Operation and Maintenance Manual while the service brakes are applied. 3. Increase the engine speed to high idle. The machine should not move. 4. Reduce the engine speed to low idle and move the transmission direction control to the NEUTRAL position. Engage the parking brake. Lower the implement to the ground. Stop the engine. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -29- Module 2 - Air Brake Systems Parking Brake Holding Ability Test The general procedure for the parking brake holding ability test involves: 1. Start the engine, allow air pressure to reach the normal operating range, raise the implement slightly, and engage the parking brake. 2. Ensure that the transmission control is in the MANUAL position. 3. Put the transmission control in the gear specified by the Operation and Maintenance Manual. 4. Increase the engine speed to high idle. The machine should not move. 5. Reduce the engine speed to low idle and move the transmission direction control to the NEUTRAL position. Lower the implement to the ground and stop the engine. NOTE: The parking brake holding ability test procedure on some machines requires the parking brake to be tested while the machine is parked on a slope. NOTE: Carry out all the necessary safety checks/precautions/procedures before attempting any task. Refer to the appropriate service literature when performing the brake holding tests. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -30- Module 2 - Air Brake Systems LEAK CHECK 1 6 5 3 2 4 30 1. Parking brake control valve 4. Governor 2. Air reservoirs 5. Service brake control valve 3. Air compressor 6. Service brake pedal OPERATIONAL CHECKS: LEAK CHECK Explain the general procedure for Air leakage checks will determine where an air leak is present and can be heard or performing the air leakage check. seen when soapy water is applied to connections, valves, and hoses. The general procedure for an air leakage check involves: 1. Start the engine and allow the air pressure to reach the maximum pressure. Stop the engine. 2. Check the following for leaks: - Parking brake control valve (1) - Valves on the air reservoirs (2) - Air compressor (3) and governor (4) - Hoses, lines, and connections from the air compressor, the air reservoirs, and service brake control valve (5) 3. Depress and hold the service brake pedal (6). Check for leakage in the hoses, lines, and connections from the service brake control valve to the service brakes. Release the pedal. 4. Release the parking brakes and check for leakage in the hoses, lines, and connections from the parking brake control valve to the parking brake. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -31- Module 2 - Air Brake Systems NOTE: If the air pressure in the air tank lowers to the cut-in pressure, start the engine and operate the engine until the air pressure reaches the maximum pressure. Stop the engine. NOTE: When checking the service and parking brake control valves for leakage, check the exhaust passages for any restrictions such as mud or dirt. Any restrictions may prevent or slow the release of air pressure during normal operation. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -32- Module 2 - Air Brake Systems AIR PRESSURE GAUGE TEST 4 1 3 2 198-4240 Digital Pressure Indicator Group 32 1. Air pressure gauges 3. Drain valve ports 2. Drain valves 4. Air gauge ports OPERATIONAL CHECKS: AIR PRESSURE GAUGE TEST Explain the general procedure The air pressure gauge test verifies the accuracy of the air pressure gauges in the for performing the air pressure cab by comparing the air pressure value to a test gauge. gauge test. The general procedure to test the air pressure gauges (1) involves: 1. Release the air pressure from the air reservoir by opening the drain valves (2). 2. Install a pressure gauge (per the Testing and Adjusting manual) at each drain valve port (3) and at the cab air gauge ports (4). 3. Start the engine and allow the air pressure to increase to the maximum pressure. 4. Record the air pressure on all gauges and note the difference between the cab air gauge pressure and the test pressure gauges. 5. If the difference is air pressure between the gauges is not within specification, the cab air pressure gauge may require replacement. The test can be performed using the 198-4240 Digital Pressure Indicator Group tool. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -33- Module 2 - Air Brake Systems AIR COMPRESSOR GOVERNOR TEST 1 33 2 198-4240 Digital Pressure Indicator Group 33 1. Governor 3. Drain valve ports 2. Drain valves OPERATIONAL CHECKS: AIR COMPRESSOR GOVERNOR TEST Explain the general procedure for The air compressor governor test verifies the governor operation. performing the air compressor governor test. The general procedure to test the air compressor governor (1) involves: 1. Release the air pressure from the air reservoir by opening the drain valves (2). 2. Install a pressure gauge (per the Testing and Adjusting manual) at each drain valve port (3). 3. Start the engine and allow the air pressure to increase to the maximum pressure. 4. Record the cut-out air pressure. 5. Depress the service brake pedal rapidly several times. 6. Record the cut-in air pressure. 7. If the cut-out or cut-in pressure is not in the correct range adjust the governor. The test can be performed using the 198-4240 Digital Pressure Indicator Group tool. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -34- Module 2 - Air Brake Systems AIR COMPRESSOR GOVERNOR ADJUSTMENT 1 3 2 34 1. Cover 3. Adjustment screw 2. Locknut OPERATIONAL CHECKS: AIR COMPRESSOR GOVERNOR ADJUSTMENT Explain the general procedure for The general procedure to adjust the governor involves: performing the air compressor governor adjustment. 1. Remove the cover (1) from the top of the governor. 2. Loosen the locknut (2) on the adjustment screw (3). 3. Turn the adjustment screw counterclockwise to increase the cut-in pressure or turn the adjustment screw clockwise to decrease the cut-in pressure. 4. If the adjustment does not change the cut-out pressure, the unloading valves in the air compressor may not be operating correctly. 5. If the difference between the cut-in pressure and the cut-out pressure is incorrect, the air compressor governor may not be operating correctly. NOTE: The charging cut-in and cut-out pressure limits change together. The range between cut-in and cutout pressure is not adjustable. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -35- Module 2 - Air Brake Systems LOW BRAKE AIR PRESSURE SWITCH TEST 1 4 3 2 198-4240 Digital Pressure Indicator Group 35 1. Low brake air pressure switch 3. Drain valve ports 2. Drain valves 4. Brake pressure indicator OPERATIONAL CHECKS: LOW BRAKE AIR PRESSURE SWITCH TEST Explain the general procedure The general procedure to test the low brake air pressure switch (1) involves: for performing the low brake air 1. Release the air pressure from the air reservoir by opening the drain valves (2). pressure switch test. 2. Install a pressure gauge (per the Testing and Adjusting manual) at each drain valve port (3). 3. Start the engine and allow the air pressure to increase to the maximum pressure. 4. The action alarm will sound and the brake pressure indicator (4) will illuminate until the pressure is above the pressure specified in the Testing and Adjusting manual. 5. Allow the air pressure to increase to the cut-out pressure. 6. Stop the engine and turn the engine start switch to the ON position. 7. Observe the test pressure gauges while depressing the service brake pedal rapidly several times to release the air pressure. 8. The action alarm must sound and the brake pressure indicator must illuminate at the pressure specified in the Testing and Adjusting manual. 9. If the alarm and/or the indicator does not operate at the correct air pressure, turn the engine start switch to the OFF position. Check the electrical connections for the alarm, the indicator, and low brake air pressure switches. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -36- Module 2 - Air Brake Systems SYSTEM PROTECTION VALVE TEST (AT) Parking Brake Schematic D400D Articulated Truck 198-4240 Digital Pressure Indicator Group 36 OPERATIONAL CHECKS: SYSTEM PROTECTION VALVE TEST (AT) Explain the general procedure for For an AT, the system protection valve test verifies the system protection valve performing the system protection operation. valve test for an AT. The general procedure to test the system protection valve (arrow) involves: 1. Start the engine and allow the air pressure to increase to maximum pressure. 2. Shut off the engine and apply a soap and water solution to the system protection valve and connections to check for leaks. Repair any leaks before continuing with this test. 3. Recharge the air system if necessary and then shut off the engine. 4. Install a pressure gauge at the parking brake port (per Test & Adj manual). 5. With the engine OFF depress the service brake pedal and observe the air pressure gauges in the cab. The service brake pressure should decrease, but the parking brake pressure should remain at full pressure. 6. Recharge the air system if necessary and then shut off the engine. 7. Operate the parking brake lever several times and observe the air pressure gauges. The air pressure should decrease to the pressure specified in the Testing and Adjusting manual and stabilize at this pressure. 8. Recharge the air system if necessary and then shut off the engine. 9. Carefully loosen the system protection valve air line, as specified in the Testing and Adjusting manual, until air begins to leak. The parking brake air pressure should remain at full pressure and the service brake air pressure should decrease to the pressure specified in the Testing and Adjusting manual. 10. If any of the pressures are out of range, the system protection valve may need to be repaired or replaced. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -37- Module 2 - Air Brake Systems PARKING BRAKE AIR PRESSURE TEST 198-4240 Digital Pressure Indicator Group 37 OPERATIONAL CHECKS: PARKING BRAKE AIR PRESSURE TEST Explain the general procedure for The general procedure to check the parking brake air pressure involves: performing the parking brake air pressure test. 1. Install a pressure gauge (per the Testing and Adjusting manual) at each drain valve port (arrows). 2. Start the engine and allow the air pressure to increase to the cut-out pressure. 3. Place the parking brake knob in the DISENGAGED position. Stop the engine. 4. Depress the brake pedal rapidly several times until the knob automatically moves to the ENGAGED position and record the pressure. 5. If the pressure recorded is not correct further parking brake system testing and inspection may be required. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -38- Module 2 - Air Brake Systems SERVICE BRAKE AIR PRESSURE/OIL PRESSURE TEST 1 3 2 3 198-4240 Digital Pressure Indicator Group 38 1. Brake cylinders 3. Rear slack adjuster 2. Slack adjuster test port OPERATIONAL CHECKS: SERVICE BRAKE AIR PRESSURE AND OIL PRESSURE TEST Explain the general procedure for The general procedure to check the service brake air pressure and oil pressure in performing the service brake air an air over hydraulic brake system involves: pressure and oil pressure test. 1. Move the parking brake control to the ON position to remove all the air pressure from the service brakes air circuit. 2. Install a pressure gauge (per the Testing and Adjusting manual) at the top the brake cylinders (1) and slack adjusters test ports (2). The rear slack adjuster (3) is shown. 3. Use an external air supply to charge the air system. 4. Start the engine and operate at high idle. 5. Apply the service brake and record the air pressure at each brake cylinder and the oil pressure at each slack adjuster. 6. If the pressure recorded is not correct further service brake system testing and inspection may be required. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -39- Module 2 - Air Brake Systems HYDRAULIC CYLINDER STROKE TEST 1U-9360 Displacement Gauge Assembly 39 OPERATIONAL CHECKS: HYDRAULIC CYLINDER STROKE TEST Explain the general procedure for The general procedure to check the hydraulic cylinder stroke in an air over performing the hydraulic cylinder hydraulic brake system involves: stroke test. 1. Install a displacement gauge and rod (per the Testing and Adjusting manual). 2. Ensure the air system is fully charged. 3. With the service brake fully released, push the gauge rod into the hydraulic cylinder (arrow) until it contacts the piston. 4. Measure and record the rod length. 5. Fully depress the brake pedal and push the rod back into contact with the piston and measure and record the rod length. 6. The difference between the two measurements represents the hydraulic cylinder stroke. 7. If the stroke is less than or more than the specification in the Testing and Adjusting manual there may be a fault in the brake system and further service brake system testing and inspection may be required. The test can be performed using the service tool 1U-9360 Displacement Gauge Assembly. (Formerly available as FT1097.) Caterpillar: Confidential Yellow SERVxxxx - 12/24 -40- Module 2 - Air Brake Systems SLACK ADJUSTER TEST 8S-2263 Valve Spring Tester 40 OPERATIONAL CHECKS: SLACK ADJUSTER TEST Explain the general procedure The general procedure to test a slack adjuster (arrow) in an air over hydraulic brake for performing the slack adjuster system involves: test. 1. Ensure the air system is fully charged. 2. Fully apply and release the service brakes. 3. As soon as the brakes are released, open the brake purge screw. If oil pressure is present, the slack adjuster for that brake assembly is operating correctly. Repeat the procedure for each wheel brake assembly. 4. If there is no pressure at the purge screws further brake system testing and inspection may be required. NOTE: On some machines, the oil pressure can be checked at the slack adjuster pressure ports and compared to the oil pressure at the wheel brake assemblies without having to open the brake purge screws. NOTE: The slack adjuster can also be tested off the machine using an 8S-2263 Valve Spring Tester. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -41- Module 2 - Air Brake Systems PARKING BRAKE OIL PRESSURE TEST 198-4240 Digital Pressure Indicator Group 41 OPERATIONAL CHECKS: PARKING BRAKE OIL PRESSURE TEST Explain the general procedure for The general procedure to check the parking brake oil pressure in an air over performing the parking brake oil hydraulic brake system involves: pressure test. 1. Install a pressure gauge (per the Testing and Adjusting manual). 2. While the parking brakes are activated, start the engine and operate at high idle rpm with the transmission in NEUTRAL. 3. The oil pressure must be at the relief valve setting as specified in the Testing and Adjusting manual. 4. Turn the adjustment screw counterclockwise to decrease the parking brake oil pressure or clockwise to increase the parking brake oil pressure. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -42- Module 2 - Air Brake Systems SYSTEM TROUBLESHOOTING OVERVIEW 42 SYSTEM TROUBLESHOOTING OVERVIEW Give an overview of system To isolate the root cause of a problem, further inspection of system components troubleshooting. and/or system testing may be required. A machine that does not brake efficiently may indicate a problem in the brake system. This section identifies the component inspections, as well as the types of tests that should be performed to troubleshoot problems with the air brake system. Prior to inspection and component testing, using a troubleshooting chart can help establish possible causes based on any symptoms that may have been identified. After completing this section, you should be able to: Describe the process for troubleshooting air brake system problems Identify possible causes of an air brake system issue based on given symptoms Describe inspection procedures to find the root cause of an air brake system issue NOTE: Equipment inspections identify potential problems before failure and reduce unplanned downtime. Family and model-specific checklists have been developed for technicians and are available in SIS or at https://dealer. cat.com. TA2 inspections are either performed annually or when required. Such inspections can pinpoint problems in the brake system. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -43- Module 2 - Air Brake Systems CATERPILLAR 7-STEP DIAGNOSTIC PROCESS 43 CATERPILLAR 7-STEP DIAGNOSTIC PROCESS Explain that this module will When approaching brake system service issues, a consistent diagnostic strategy focus on the Investigate and should be followed. Isolate steps of the Caterpillar 7-Step Diagnostic Process. The Caterpillar 7-Step Diagnostic Process involves investigating, isolating, repairing, and documenting the customer concern. Each step accumulates information that helps direct the next step. Following is an example summary of the steps involved for a brake system issue: Step 1: If a customer is concerned about erratic brake operation, make a visual inspection to identify symptoms that could indicate the brake system is the cause. Step 2: During the inspection, look for signs of any external leaks. Look for signs of damage to any of the brake system components. Step 3: The results of Steps 1 and 2 will guide development of a list of possible causes. This might include air or oil restrictions or leaks somewhere in the brake air system. Step 4: The root cause may involve performing tests to determine air pressure. Test results may require closer examination of components and fittings for leaks, cracks, or failures. Step 5: When repairing the root cause of the concern, remember the air brake system must maintain proper air pressure to perform correctly. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -44- Module 2 - Air Brake Systems Step 6: When verifying repairs to the brake system, always check for the proper air pressure. Then, ensure there are no external leaks. Finally, compare the results to TMI specifications. Step 7: To properly document the repair, describe the concern, root cause, service details, and part number (if applicable). Any additional information that might be helpful in future analysis or repairs should also be included. In this module, the focus will be on the Investigate and Isolate steps to identify the possible cause(s) using inspection and testing procedures. NOTE: Always refer to Caterpillar service information for the correct tests and specifications for the machine being serviced. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -45- Module 2 - Air Brake Systems INITIAL INSPECTION 6 1 5 2 3 4 45 1. Air compressor 4. Brake assemblies 2. Governor 5. Air reservoirs 3. Brake lines and fittings 6. Brake control valve SYSTEM TROUBLESHOOTING: INITIAL INSPECTION Give a basic explanation of Performing an external inspection of the air brake system occurs during the initial performing an external inspection inspection, after the customer concern has been verified. during the initial inspection step. Prior to the inspection, lower the implements to the ground and then stop the engine Check the following components for damage and leaks: Air compressor (1) Governor (2) Brake lines and fittings (3) Brake assemblies (4) Air reservoirs (5) Brake control valve (6) Check for loose connection and any restrictions including: Sharp bends Clamps that are incorrectly installed Damaged hoses Damaged air lines Caterpillar: Confidential Yellow SERVxxxx - 12/24 -46- Module 2 - Air Brake Systems SYMPTOM TROUBLESHOOTING 46 SYMPTOM TROUBLESHOOTING Review the troubleshooting A troubleshooting chart helps establish a list of possible causes based on the chart included in the Instructor symptoms that may have been identified while verifying the customer concern and Materials and Activity Workbook. conducting an initial inspection. Additionally, the Testing and Adjusting manual or Troubleshooting manual may provide probable causes to known problems, as Inform the participants that the shown in the example above. intent of the troubleshooting chart is to teach diagnostic principles. It is not intended to teach all possible symptoms, nor encourage jumping to conclusions. For additional troubleshooting details read current service information. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -47- Module 2 - Air Brake Systems COMPRESSOR 47 SYSTEM TROUBLESHOOTING: COMPRESSOR Identify symptoms that could Either of the following symptoms could indicate a problem with the air compressor. indicate a problem with the air compressor. Service brakes do not engage Parking brakes do not release The operation of an air compressor involves sending air under pressure to the service and parking brake air systems. If the air compressor is suspected as a possible problem, perform the Air Compressor Governor Test covered in the System Testing and Adjusting section to determine if the air compressor could be causing the problem. If the results show that the pressure is not within specifications, then service or replace the air compressor as needed. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -48- Module 2 - Air Brake Systems GOVERNOR 48 SYSTEM TROUBLESHOOTING: GOVERNOR Identify symptoms that could Either of the following symptoms could indicate a problem with the governor. indicate a problem with the governor. Service brakes do not engage Parking brakes do not release The operation of governor involves controlling the amount of air flow in the air system. If the master cylinder is suspected as a possible problem, perform the Air Compressor Governor Test covered in the System Testing and Adjusting section to determine if the governor could be causing the problem. If the results show that the pressure is not within specifications, then adjust or replace the governor as needed. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -49- Module 2 - Air Brake Systems AIR DRYER 49 SYSTEM TROUBLESHOOTING: AIR DRYER Identify symptoms that could The following symptom could indicate a problem with the air dryer. indicate a problem with the air dryer. Erratic brake operation due to moisture in the brake system The operation of the air dryer involves removing the moisture, carbon, and oil from the compressed air before entering the air reservoir. If the air dryer is suspected as a possible problem, inspect the air dryer for leaks or damage and replace the air dryer as needed. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -50- Module 2 - Air Brake Systems AIR RESERVOIR 50 SYSTEM TROUBLESHOOTING: AIR RESERVOIR Identify symptoms that could Either of the following symptoms could indicate a problem with the air reservoir. indicate a problem with the air reservoir. Service brakes do not engage Parking brakes do not release The function of the air reservoir involves storing and supplying air to the brake system. If the air reservoir is suspected as a possible problem, drain the moisture from the reservoir and inspect for leaks or damage. Replace the air reservoir as needed. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -51- Module 2 - Air Brake Systems SERVICE BRAKE CONTROL VALVE 51 SYSTEM TROUBLESHOOTING: SERVICE BRAKE CONTROL VALVE Identify symptoms that could Any of the following symptoms could indicate a problem with the service brake indicate a problem with the control valve. service brake control valve. Service brakes do not operate Service brakes are not applied evenly Brakes do not release The operation of a service brake control valve involves sending air under pressure to the brake assemblies. If the service brake control valve is suspected as a possible problem, check service brake circuit air pressure to determine if the service brake control valve could be causing the problem. If the results show that the pressure is not within specifications, then service or replace the service brake control valve as needed. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -52- Module 2 - Air Brake Systems SERVICE BRAKES Multi-disc Brake Air Brake Chamber 52 SYSTEM TROUBLESHOOTING: SERVICE BRAKES Identify symptoms that could Either of the following symptoms could indicate a problem with the multi-disc indicate a problem with the service brake assembly or an air brake chamber: service brakes. Service brakes do not engage Service brakes do not release The operation of the service brake assembly involves using friction to cause the machine to slow down and stop. If the service brake assembly is suspected as a possible problem, perform the Service Brake Holding Ability Test covered in the System Testing and Adjusting section. If the machine moved during the service brake test, then inspect the service brake components for damage and excessive wear. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -53- Module 2 - Air Brake Systems BRAKE CYLINDER 53 SYSTEM TROUBLESHOOTING: BRAKE CYLINDER Identify symptoms that could Either of the following symptoms could indicate a problem with the brake cylinder in indicate a problem with the brake an air over hydraulic brake system: cylinder. Service brakes do not engage Service brakes do not release The operation of a brake cylinder involves supplying compressed oil to the service brakes. If the a brake cylinder is suspected as a possible problem, perform the Hydraulic Cylinder Stroke Test covered in the System Testing and Adjusting section to determine if the brake cylinder could be causing the problem. If the results show that the brake cylinder is not within specifications, then service or replace the brake cylinder as needed. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -54- Module 2 - Air Brake Systems SLACK ADJUSTER 54 SYSTEM TROUBLESHOOTING: SLACK ADJUSTER Identify symptoms that could Either of the following symptoms could indicate a problem with the slack adjuster in indicate a problem with the slack an air over hydraulic brake system: adjuster. Service brakes do not operate Service brakes are not applied evenly The operation of a slack adjuster involves compensating for brake disc wear by allowing a small volume of oil to flow through the slack adjuster and remain between the slack adjuster and the brake piston under low pressure. If a slack adjuster is suspected as a possible problem, perform the Slack Adjuster Test covered in the System Testing and Adjusting section to determine if the slack adjuster could be causing the problem. If the results show that the slack adjuster is not within specifications, then service or replace the slack adjuster as needed. Caterpillar: Confidential Yellow SERVxxxx - 12/24 -55- Module 2 - Air Brake Systems PARKING BRAKE CONTROL VALVE 55 SYSTEM TROUBLESHOOTING: PARKING BRAKE CONTROL VALVE Identify symptoms that could Either of the following symptoms could indicate a problem with the parking brake indicate a problem with the control valve: parking brake control valve. Parking brakes do not engage Parking brakes do not release The operation of a parking brake control valve involves controlling the air to engage or release the parking brakes. If the parking brake control valve is suspected as a possible problem, check parking brake circuit air pressure to determine if the parkin